The present invention relates to methods and systems for designing components that, in use, are expected to encounter vibratory loading. In particular, the invention has applicability in the field of aero engines and turbomachinery.
A typical turbine engine (or typical turbomachinery) contains several annular fluid-conveying ducts which form, e.g., the compressor or turbine sections of the engine.
Each of these ducts contains a number of blades or vanes (which are henceforth called guide vanes or simply vanes) circumferentially distributed in one or more vane assemblies in the duct. The vanes are categorized as rotor blades or stator vanes depending on whether or not the respective vane rotates in the duct when the engine operates. Typically, each vane has a surface that extends radially across the duct.
When subjected to high speed airflows, vanes typically become susceptible to large amplitude vibration, often caused by unsteady aerodynamic forces. The unsteady aerodynamic forces are caused by the “wakes” of components upstream in the fluid-conveying duct. This is illustrated by the following example in which the vane of interest is a rotating compressor blade. Note that the vanes can also be subjected to forcing from the downstream pressure fields (e.g. from downstream vanes).
In this example, a set of stator blades are provided, situated upstream of the compressor blade. The function of the stator blades is to adjust the direction of fluid flow towards the compressor blades. Each stator blade has an associated wake. As the compressor blade rotates, it “sees” a wake from each stator blade in turn. The frequency f at which the compressor blade sees a stator wake is therefore equal to the number of complete revolutions of the compressor blade per second multiplied by the number of stator blades in the set of stator blades (assuming that the stator blades are circumferentially equispaced from each other).
The travel of the compressor blade through the wakes of the stator blades causes varying aerodynamic forces to act on the compressor blade. The frequency of variation of these forces is f. Vibration of the blade resulting from the varying aerodynamic forces is termed the “forced response”. If f matches a natural frequency of a mode of vibration of the compressor blade and if the unsteady aerodynamic forcing matches the associated modeshape, then the compressor blade will resonate. Consequently, the amplitude of vibration of the compressor blade will be relatively large. Such vibrations are detrimental to the compressor blade, and may result in failure of the blade. It is desirable to reduce the detrimental effect of forced response in order to avoid failure of components in the aero engine or turbomachinery of interest.
A possible solution to this problem is to avoid the vane's resonant frequencies by operating at other frequencies. However, this is difficult to implement in practice without altering the performance (i.e. efficiency) of the engine or turbomachinery.
Another possible solution is to decrease the forcing on the vanes, i.e., in the example above, by decreasing the variation in aerodynamic forces seen by the compressor blade. One way to this would be to locate the compressor blades further downstream of the stator blades. However, this increases the axial length of the engine/machinery and thus increases the total weight. This is undesirable.
Another possible solution is to increase the damping available to the vane. However, this is difficult to introduce and control.
The inventors have realised that the above problem may be addressed by altering the vane of interest in order to alter its forced response at a particular frequency. In. particular, they have realised that the problem may be addressed during the design stage of the vane.
When designing a vane, it is possible to- determine the local modal force on the vane at a particular frequency of aerodynamic unsteady pressures. The local modal force MFL provides a measure of the likely forced response of a vane at that local element.
MFL is the scalar product of two vector quantities, ΔFL and ΔxL. ΔFL is the local unsteady aerodynamic force acting on the surface of the local element of the vane. ΔxLis the modeshape of the local element of the vane. The modeshape is the actual vibrating shape of a vane at a natural frequency. The modeshape is a physical property of the vane which depends on its geometry and the material which makes it.